Microlens screens, photopolymerisable materials and artifacts utilising the same

ABSTRACT

There are disclosed variations on a basic technique for forming a diffusion or rear-projection screen comprising an array of microlenses formed by selective light exposure and consequent selective polymerization of a sheet of photopolymerizable material. Thus, the lenses may be made elongate in a preferred direction transverse to the lens axes by exposure through a mesh formed with an array of slots. The lenses may be graded refractive index lenses having a surface relief adding to the lens power, such relief being formed naturally or by molding. The photopolymerizable material may be exposed by laser light, for example to a holographic pattern and such exposure may be made through a quarter-wave plate continuously rotated to eliminate micro defects. Enhanced lens powers may be achieved by heating the photopolymerizable material to close to the softening temperature of the associated polymer before exposure to light and maintaining the material at that temperature during the resultant polymerization.

THIS INVENTION relates to light diffusing screens and other artefactscomprising arrays of microlenses, to methods of making such screens andartefacts or light diffusing materials therefor to the manufacture ofsuch artefacts and materials using photopolymerisable materials, to thefabrication of microlenses and other microstructures, and to themanufacture of graded refractive index lenses and other products. Theinvention also relates to an optical printing method and apparatuscapable of being realised using a component formed of graded refractiveindex material.

Our published British Patent Application GB2206979A (8812644.6)discloses, inter alia, a technique for forming a microlens diffusingscreen, for example for television, which comprises a transparent sheetbearing an array of microlenses, and in which the microlenses are in theform of graded refractive index lenses formed by selective exposure of aphotopolymer to a pattern of dots, such exposure being in ultra-violetlight or suitable visible light and being effected by "contact" exposurethrough a mask having an array of circular holes formed therein, or byspot-by-spot exposure using a scanned ultra-violet laser, or byultra-violet holography.

It is one object of the present invention to provide an improvement inthe above-noted technique disclosed in GB2206979A whereby such a screenmay be arranged to disperse light substantially more in one dimensionparallel to the plane of the screen than in a perpendicular direction,that is to say, whereby the screen will have different polardistribution for light diffused thereby, about two mutuallyperpendicular axes. Thus, for example, as applied to a television screenonto which light is projected from a luminous source such as a cathoderay tube, it may be arranged that the range of angles about a verticalaxis relative to the screen over which the screen will appear acceptablybright to an observer will be substantially wider than the correspondingrange about a horizontal axis, thereby utilising more efficiently theimage-forming light directed to the screen from the luminous source,bearing in mind that, in a typical viewing situation in which aplurality of observers are viewing the screen simultaneously, or inwhich a single observer may view the screen from various positions, thescreen will generally be approximately at the eye-level of the observer,but the observer will not necessarily be in front of the screen.

According to one aspect of the present invention, the individualmicrolenses in the array are elongated in one direction in the plane ofthe screen, so that a middle region of each microlens acts as acylindrical lens whilst the end regions thereof act as respective partsof respective spherical lenses.

The terms "cylindrical" and "spherical" used above are used merely todescribe the mode of operation of the lenses. The lenses themselves arepreferably, as in GB2206979A, formed by graded refractive index effectsin a photopolymer layer bounded by substantially planar surfaces, or bysurfaces having some surface relief effects as an artefact or by-productof the method used to form the graded refractive index lenses.

The present invention, in another aspect, concerns various other usesfor a microlens screen or analogous light-transmitting material,hereinafter referred to as "the microlens material", in the form of atransparent polymeric sheet incorporating an array of graded refractiveindex microlenses. Such a microlens screen acts as a very efficient rearprojection/diffusion screen exhibiting reduced scatter of light toundesired angles and a reduction in the "hot spot" effect.

In one class of such use, the microlens material performs astraightforward diffusion or light-directing function independently ofany projection or optical system. For example, such sheet material maybe applied as a film to glass used architecturally, e.g. for glazingwindows and doors, or may, for such a purpose, be sandwiched between twosheets of glass or rigid transparent plastics to afford a more refinedversion of the known "frosted" or surface-textured glass commonly usedin such applications. Such microlens material, supported in such mannerby rigid glass or plastics transparent sheets may also be used as"privacy glass" for example in bathroom shower cubicles and the like.

Such microlens material may also be applied to, or incorporated in,diffusing structures or panels for, for example, electric lamps, indomestic or public buildings. In this context, use may be made of thepossibility, as disclosed in Application No. 8812644.6, of controlledorientation of the substantially "cylindrical" graded refractive indexlenses in such material to achieve preferential direction of light froma lamp or other light source in a particular direction. Such materialmay also have application in the automotive field, for example inillumination of dashboard displays or as an adjunct to the normalvehicle headlamps or tail-lamps, for example, to improve thedirectionality of such lamps and reduce the scattering of light fromsuch lamps in directions not required by the driver of the vehiclehaving the lamps and which tends to dazzle, or be a distraction orirritant to drivers of other vehicles.

Such material may also, of course, be utilised in image projectionapplications other than TV and the like systems, for example in thefocusing screens of reflex cameras, in slide and film viewers fordomestic and other use, where ground glass or correspondingly texturedacrylic screens are conventionally used, in analogous industrialprojection systems, and so on. The polymeric sheet materialincorporating the microlenses lends itself well to the construction oflarge-scale video displays as the sheet material can readily be cut andtrimmed to form respective sections which are suitably supported, inabutment with one another, to form a composite large-scale rearprojection screen. In all of these applications, the ability of themicrolens material, suitably configured, to ensure that most of thelight passing through the microlens screen is directed to the region atwhich the viewer's eye is or may be located, and that little isscattered to angles outside the viewing area, provides a subjectivelybright image suitable for "daylight" viewing.

When the material is used in c.r.t., TV and VDU applications in whichindividual picture units or "pixels" appear at predetermined locationsin the screen area, the arrangement may be such that a respectivemicrolens is allocated to and aligned with each "pixel", affordingenhancement of viewing. Furthermore, it is possible, in computer monitorand the like applications in which respective areas of the screen formpredetermined character display areas, for each such character area tobe allocated a respective microlens. (In this instance, the microlensesmay be of correspondingly large scale).

A microlens screen comprising such polymeric microlens material may bedetachably mounted on the front of a VDU, so as to be interchangeablewith other such microlens screens with microlenses of differentcharacteristics, array pitches, lens diameters, etc. suited to selecteddisplay types.

In such applications, the microlens material may be supported on a rigidplanar transparent glass sheet arranged in front of the c.r.t. screen(in which case the flat sheet, used in conjunction with a conventionaloutwardly convex c.r.t. screen may touch the latter screen at the centrebut will be spaced somewhat therefrom at locations further from thecentre and will be spaced furthest from the c.r.t. in the region of thecorners of the screen.

Alternatively, the polymeric microlens sheet material may be detachablyadhered directly to the VDU screen. Thus, for example, the VDU screenmay be arranged to have a cylindrical screen surface and the polymericmicrolens sheet material may be wrapped around the cylindrical surfaceof the screen and adhered directly thereto.

It is further contemplated that an array of multiple-element microlensesmay be formed by sandwiching together a plurality of polymeric sheets,each with a respective array of graded refractive index microlensesthereon, so that each microlens in one sheet is superimposed withcorresponding microlenses in the other sheet of the sandwich, each setof superimposed "simple" microlenses together forming a respectivecompound or multiple-element microlens. The polymeric sheet material, orsuch a sandwich of polymer sheets, may be applied to or sandwiched witha liquid crystal display arrangement, such as a multi-pixel TV or VDULCD screen.

Such microlens material may also be used, in certain applications, inconjunction with fibre optics.

It is further envisaged that the polymeric microlens material may beused to advantage in high definition TV systems, to optimise theperformance of the latter.

It is an object of the present invention, in yet another of its aspects,to provide an improved microlens screen or light-diffusing sheetmaterial.

According to this aspect of the invention there is provided a sheet oftransparent material formed with an array of integral graded refractiveindex microlenses wherein each microlens terminates, on at least one ofthe surfaces of the sheet, in a surface relief formation which adds tothe power of the respective microlens.

The invention also includes, within its scope, a single gradedrefractive index lens having at least one of its end surfaces formed asa convex or concave lens surface.

In a preferred embodiment described in GB226979, to which referenceshould be had, the graded refractive index lenses are formed in a layerof a photopolymer by selective polymerisation of a photopolymerisableresin, such polymerisation having been produced by correspondinglyvarying the exposure to light of the layer of resin, (or at least thecorresponding layer of the corresponding monomer) over the area of thesheet, during manufacture. In the preferred method, disclosed in GB2206979, the selective exposure is achieved by exposing themonomer/photopolymer layer to ultraviolet light through a mask, havingan array of holes therethrough corresponding to the desired array ofmicrolenses.

The present invention, in another of its aspects, concerns variants ofthe preferred form of screen disclosed in GB 2206979 and of the materialused to form such screen and novel methods which may be utilised for themanufacture of such variants.

It is among the objects of the present invention to provide alight-diffusing material of enhanced power.

Thus, according to another aspect of the invention there is provided amethod of making a light diffusing material comprising a sheet oflight-transmitting base material, having a surface thereof configured toafford an array of microlenses, coating the base material with a layerof a medium, such as a photopolymer, of which the refractive index canbe varied by exposure to light, then exposing the variable refractiveindex medium to light through the said base material having saidmicrolenses formed thereon, whereby the localised variation in intensityof said light, through the variable refractive index medium, produced bysaid microlenses, causes corresponding variations in refractive index insaid medium whereby there is produced, in said medium, an array ofgraded refractive index lenses each aligned with, and adding its opticaleffect to, a respective said microlens of the base material.

According to yet another aspect of the invention there is provided amethod of making a light diffusing material comprising forming a sheetor layer of a medium, such as a photopolymer, of which the refractiveindex can be varied by exposure to light, one surface of said sheet orlayer being moulded or embossed to afford an array of microlenses,exposing the sheet or layer of said medium to light through said mouldedor embossed surface, whereby the localised variation in said light,through the variable refractive index medium, produced by refraction atsaid moulded or embossed surface, causes corresponding variations inrefractive index in said medium, whereby there is produced, in saidmedium, an array of graded refractive index lenses each aligned with,and adding its optical effect to, a respective microlens surface definedby said moulding or embossing.

According to a further aspect of the invention there is provided a lightdiffusing screen comprising a transparent base sheet which has a Fresnellens or prism formed on one surface thereof by a moulding or embossingstep, and which is coated, on the opposite surface thereof, with a layerof a variable refractive index material incorporating an array of gradedrefractive index lenses.

In the preferred method of forming a microlens screen disclosed in ourApplication No. 8812644.6, the graded refractive index lenses are formedby selective exposure of the photopolymeric material, (or rather of thecorresponding monomer) to laser light, for example from an ultra-violetlaser.

The applicants have found that, in an array of graded refractive indexmicrolenses formed by the above-noted method, using laser light forexposure of the photopolymer, the microlenses formed have opticaldefects in the form of a microstructure, within the lens, constituted byhighly localised variations in refractive index. It has been discovered,by the inventor of the present invention, that such microstructurewithin the microlenses is attributable to polarisation of the laserlight.

It is an object of the present invention, in yet another aspect, toprovide an improved method of manufacturing a microlens array of thekind referred to by which the above-noted defects, due to suchmicrostructure, can be avoided.

According to the last-noted aspect of the present invention there isprovided a method of manufacturing a microlens screen of the typespecified, by selective exposure of the photopolymer, or thecorresponding monomer, to laser light, wherein a quarter wave plate isinterposed between the source of laser light and the photopolymer andwherein, during exposure, the quarter wave plate is rotated orrotationally oscillated, about an axis perpendicular to its oppositefaces, (i.e. perpendicular to the fast and slow axes), whereby, duringexposure, the polarisation of the laser light within the photopolymer iscontinuously varied, to eliminate micro-defects due to suchpolarisation.

The source of laser light may comprise a scanning laser arranged toexpose the areas of the polymer in which microlenses are to be formed ona spot-by-spot basis through an appropriate optical mask formed with thecorresponding array of circular light-transmitting apertures asdiscussed In British Patent Application No. 8812644.6.

In the preferred method disclosed in our Application No. 8812644.6, thegraded refractive index lenses are formed by exposure of thephotopolymeric material to laser light, for example from an ultra-violetlaser, through a clear-dot mask, the arrangement being such that thephotopolymer layer is scanned, through the mask, by the laser beam.

It is yet another object of the present invention in another of itsaspects, to provide an improved method of forming such a gradedrefractive index microlens screen.

According to this aspect of the invention, there is provided a method ofmaking a microlens screen comprising a layer of transparentphotopolymeric graded-refractive index material in which the array ofmicrolenses in the screen are formed as graded refractive index lenses,the method comprising forming holographically in a layer of saidphotopolymer or the corresponding monomer, an image of a varying lightpattern corresponding to the desired array of microlenses.

The size of the area of the photopolymer which can conveniently beexposed in the above manner in a single exposure, although large enoughto contain a large number of such microlenses, is still relativelysmall, (in relation, for example, to the size of screen desirable as arear projection screen for a rear-projection television system), andaccordingly, in a preferred embodiment of the invention, an extendedarea of the photopolymer layer is exposed in a succession of adjoiningblocks, in a scanning mode, so that successive rows of such adjoiningblocks are exposed one after the other, the adjoining blocks in eachsaid row being exposed one after the other in successive scanning steps.

GB 2193344 discloses, in the production of monolithic integratedcircuits, a technique for exposing a photoresist-coated silicon slice tothe optical image of a desired circuit pattern, in which the image onthe photoresist is produced holographically, in laser light, from ahologram embodied in a variable-refractive index photopolymer coating onthe hypotenuse face of a right-angle prism, the resist-coated siliconslice being positioned immediately adjacent said hypotenuse face and thelaser light being projected towards that hypotenuse face from within theprism, through one of the other faces of the prism. The manner in whichthe hologram is initially produced in the photopolymer layer on thehypotenuse face of the prism is likewise disclosed in GB 2193344 andutilises a "mask" bearing the desired circuit pattern and which islocated adjacent to the photopolymer layer on said hypotenuse faceduring creation of the hologram.

The present invention proposes to utilise a modification of thetechnique of GB 2193344 for the production of an array of gradedrefractive index microlenses in a layer of a graded refractive indexphotopolymer. In the technique of the present invention, the hologram onthe hypotenuse face of the prism is produced, in the same manner asdisclosed in GB2193344 but using, in place of a mask bearing a desiredcircuit pattern, a mask affording an array of transparent "holes" (themask being otherwise opaque), said array corresponding with the desiredarray of microlenses in the finished product.

It is another object of the invention to provide a method of producingmicrolenses of enhanced power by selective photopolymerisation ofcertain transparent synthetic materials. The invention affords, interalia, an improved method of manufacturing a screen of transparentmaterial incorporating an array of integral microlenses, such a screenbeing herein referred to, for convenience, as a "microlens screen".

There is disclosed in WO80/09952 a method of making a microlens screenby selective graded exposure to light of a layer of a photopolymerisablevariable refractive index substance, such as acrylamide, to produce anarray of microlenses which are in the form of graded refractive indexlenses.

Thus, according to yet another aspect of the invention, microlenses ofenhanced optical power are produced in a photopolymerisable substance byraising the temperature of said substance to close to the softeningtemperature of the associated photopolymer before selective polymerisingexposure of the substance, then selectively exposing the substance tolight whilst maintaining the substance at an elevated temperature andpermitting selective polymerisation of the substance, at an elevatedtemperature before cooling or allowing to cool.

The invention may be applied to the production of a microlens screen byproviding the transparent photopolymerisable substance as an extendedlayer and exposing said layer, in the exposure step, to a light patterncomprising an array of dots, to provide a corresponding array ofmicrolenses in said layer by the corresponding selectivephotopolymerisation of the substance.

It is thought that the enhancement of the lens power resulting from theinvention is related to the softening of the photopolymer as it isformed and an increased mobility of the as yet unpolymerised, or onlypartially polymerised components, but it is not intended to limit theinvention to any particular theory as to the mechanism involved.

In a preferred embodiment of the invention a layer of a substance, suchas acrylamide monomer, capable of selective graded photopolymerisationunder correspondingly graded exposure to ultraviolet light, is providedon a supporting substrate, which itself forms or carries an optical maskconsisting of an array of light-transmitting areas of "holes" in anopaque field. The supporting substrate may comprise a stable transparentplastics film to which the monomer layer is applied and which, ineffect, is an exposed and developed silver halide photographic film andthus carries, on its face remote from the monomer layer a gelatin layerbearing silver grains defining said opaque background. The photographicfilm has been exposed, in conventional manner, by way of a masterscreen, to the desired optical pattern comprising an array of dots, theexposed film having been subsequently developed, conventionally to formthe mask associated with the monomer layer. The exposure and/ordevelopment of the silver halide film may take place before or after thecoating of the stable plastics film with the acrylamide monomer.

The composite material, comprising the monomer layer on the supportingsubstrate with optical mask is first heated to a temperature close tothe softening temperature of the corresponding polymer (e.g.polyacrylamide) and is then exposed to ultraviolet light, through themasking layer, to bring about the desired selective polymerisation. Theelevated temperature is preferably maintained while such polymerisationproceeds, the material being thereafter allowed to cool. The product maythen be submitted to a conventional photographic bleaching process toremove the silver from the masking layer and render the latter whollytransparent. However, for some applications, the regions between the"holes" in the masking layer may be kept opaque to absorb unwantedscattered or reflected light, for example where the microlens screen isintended for a television screen.

As one surface of the monomer layer is free during exposure andphotopolymerisation, the layer is also free to undergo an associatedsurface distortion at that free surface, in the form of a convex "bump"or dome above each polymerised region. Depending upon the conditions,and the photopolymerisable substance, there may also be a variation inrefractive index accompanying the selective photopolymerisation,contributing a graded refractive index converging lens component to eachmicrolens in addition to the component due to said surface distortion,so that the optical converging effects of the surface distortion and therefractive index variation enhance one another. It is assumed, in theabove, that the microlenses in question are converging lenses, but if anarray of diverging microlenses were produced by the same technique, anyrefractive index variation would also be the converse of that occurringin the production of an array of converging microlenses so that thesurface distortion and any refractive index variation would again bothenhance the power of the microlenses.

It will be appreciated that the present invention may be applicable bothto systems where the effect of the microlenses is derived substantiallyfrom graded refractive index variation, with no significant contributionfrom surface relief effects, i.e. where the microlenses aresubstantially graded refractive index lenses and to systems where theeffect of the microlenses is derived substantially from the surfacerelief effect with no significant contribution from graded refractiveindex effects, as well as to systems where both effects make asignificant contribution.

It is envisaged that the monomer-coated silver halide film materialwould be produced, on a volume production basis, in the form of acontinuous length, which would pass longitudinally through variousstations at which the respective steps of the production method would becarried out. Thus, in such a method, the monomer-coated film would passthrough an exposure station at which ultraviolet light would be directedonto the medium from the side nearest the masking layer, and at whichradiant heat, for example from infra-red sources, would be directed ontothe opposite surface of the medium in order to maintain themonomer/photopolymer layer at the desired temperature during and afterexposure. Further radiant heat sources might be disposed slightlyupstream of the exposure station to raise the temperature of the monomerbefore passing into the exposure station. Subsequent steps in theprocess would then be carried out in successive stations following theexposure station in a manner which will be evident to those skilled inthe art.

It is an object of the present invention, in yet another of its aspects,to provide an improved method of fabricating optical microstructures,such as an array of spherical or cylindrical microlenses or Fresnellenses or prisms, or analogous reflective structures.

According to this aspect of the invention, there is provided a method ofproducing an optical microstructure, comprising the steps of:

providing a photographic material comprising a silver halide emulsionlayer on a supporting substrate,

exposing the photographic material to a desired pattern of irradiance,

processing the material to produce, in said emulsion layer, a reliefpattern corresponding to said pattern of irradiance, and using therelief-patterned emulsion surface as a master, for the production, by amoulding, embossing or the like technique, of a sheet of transparentplastics material having a corresponding or complementary surface reliefpattern forming said optical microstructure.

In one embodiment, an embossing surface on an embossing tool is derivedfrom the relief-pattern emulsion surface, and the embossing tool isthereafter used to form said optical microstructure in a sheet oftransparent plastics material by embossing a sheet of plastics materialwith said embossing tool.

Where such an embossing tool is to be formed, in order to derive saidembossing surface from said relief-patterned emulsion surface, adeformable plastics material may be applied to the relief patternedemulsion surface, using heat and/or pressure, so that a surface of saidplastics material conforms at least approximately with said reliefpatterned emulsion surface, the deformed plastics material beingsubsequently detached from the emulsion layer and the correspondingrelief surface of the deformed plastics material used for the creation,by an electroforming or electrotyping technique, of a correspondinglyrelief-patterned embossing surface, of nickel or other hard material, ofan embossing tool.

Alternatively, the relief-patterned emulsion surface may itself be usedas a mould surface to which a transparent plastics material may beapplied by a casting or moulding technique, to provide a transparentplastics sheet having a complementary surface incorporating said opticalmicrostructure, the plastics material subsequently being stripped fromthe photographic material.

Preferably said processing of the photographic material includes thestep of developing the material to convert exposed silver halide grainsin the emulsion to silver, and etch bleaching of the developed layer toproduce etch pits in the regions previously occupied by the silverimage.

It is an object of the invention in yet another of its aspects toprovide an improved optical printing system capable of rapid,high-resolution printing.

According to the invention there is provided an optical printing systemcomprising an image screen, a member providing a photosensitive surfaceand means for advancing such surface in a direction perpendicular tothat in which it faces, through an exposure zone, a plurality of lightguides each having an entrance end juxtaposed with a respective area ofsaid screen and an exit end arranged in a respective location in saidexposure zone, and means for illuminating respective said areas of saidscreen selectively as the photosensitive surface is advanced throughsaid zone, so as to expose said photosensitive surface selectively in aplurality of adjacent columns as the surface is advanced to build avisible or latent image on said surface.

In a preferred embodiment, said light guides are formed by localisedrefractive index variations in a transparent sheet of a variablerefractive index material having at least a portion thereof,accommodating said entrance ends of the light guides, overlying saidscreen and having said axis ends lying in an edge of the sheet locatedin said exposure zone.

Embodiments of the invention are described below, by way of example,with reference to the accompanying diagrammatic drawings, wherein:

FIG. 1 is a diagrammatic plan view illustrating an array of elongatedmicrolenses in accordance with one embodiment of the invention,

FIGS. 2 and 3 illustrate graded refractive index lenses, not embodyingthe invention,

FIGS. 4 and 5 are diagrammatic representations, similar to FIG. 3 butillustrating the action of lenses embodying the invention,

FIG. 6 is a schematic sectional view showing a light-diffusing materialaccording to the present invention,

FIG. 7 is a corresponding sectional view of a further form oflight-diffusing material embodying the invention,

FIG. 8 is a similar sectional view of a further light-diffusing materialembodying the invention,

FIG. 9 is a schematic perspective view illustrating the production of amicrolens screen,

FIG. 10 is a schematic sectional side view illustrating another methodwhich may be utilised in the production of a microlens screen,

FIG. 11 is a schematic plan view illustrating another aspect of themethod of FIG. 10,

FIG. 12 is a diagrammatic sectional view showing exposure of aphotographic material to an irradiance pattern through a mask,

FIG. 13 is a diagrammatic sectional view illustrating the condition ofthe material after development,

FIG. 14 is a diagrammatic sectional view illustrating the material ofFIGS. 11 and 12 after a further, etching step,

FIG. 15 is a view similar to FIGS. 12 to 14 but illustrating thedeformation of a deformable plastics material into the relief patternresulting from the etching step,

FIG. 16 is a diagrammatic sectional view illustrating the formation, byan electrotyping or electroforming technique, of an embossing toolsurface, by derivation from the deformed surface of the plasticsmaterial resulting from the step illustrated in FIG. 15,

FIG. 17 is a diagrammatic exploded perspective view illustrating theoperating principle of the invention;

FIG. 18 is a diagrammatic cross-sectional view illustrating thearrangement of light guides within a variable refractive index sheetutilised in carrying out the invention, and

FIG. 19 is a diagrammatic plan view corresponding to FIG. 18.

FIG. 1 illustrates a part of a screen according to claim 1 herein, bythe method of claim 2 herein.

The elongation of the microlenses is preferably obtained, in a processas disclosed in GB 2206979A, by exposing the photopolymer layer,supported on a substrate, to ultraviolet light, or suitable visiblelight, through a mask having an array of light-transmitting patches or"holes", the process differing from that of GB 2206979A in that thelight-transmitting patches or "holes" are elongate in correspondencewith the desired elongate form of the microlenses.

Such a mask may be formed by a photolithographic technique in which aphoto-sensitive recording layer is exposed optically to an imagecomprising an array of circular dots, the elongation of thecorresponding areas of the photosensitive layer being achieved byimparting a slight translational movement to the recording layer duringsuch exposure.

As a result, in the direction of motion of the recording layer, anelongation of each dot of the array occurs, which changes the profile ofthe "dot", which would otherwise be circular, to an elongated area withsemi-circular ends. Conventional processing of the thus-exposed maskmaterial produces a mask which, applied to the photopolymer material ina contact copying situation, automatically creates elongated microlenseswith different optical powers in orthogonal directions.

The accompanying drawing shows, diagrammatically, a portion of amicrolens screen embodying the invention, illustrating the elongatedmicrolenses. It will be appreciated that, whilst the individual lensesshown are shown as being bounded by closed lines, there will be nolines, as such, in the screen formed. The lines illustrated are merelylines connecting points of equal refractive index.

Referring to FIGS. 2 to 5 relating to embodiments of the inventionaccording to claim 4 herein, FIG. 2 shows a typical GRIN lens structure.

The parameters of a GRIN lens are specified thus:

The refractive index is engineered conventionally to be parabolic withr. Thus ##EQU1## Here n_(oo) is the refractive index on the optical axisand A is a positive constant. It is assumed in the above that therefractive index decreases with r, so that the lens has the effect of aconventional convex lens.

We define the pitch P as follows:

    P=2π/√A

If we know the pitch, we can define various imaging characteristics byvarying the length of the lens.

FIG. 3 shows a variety of imaging conditions for GRIN lenses.

As disclosed in our co-pending Patent Application No. 8812644.6, anarray of such lenses can be formed in a layer of a photopolymer such aspolyacrylamide, and varying the length of each GRIN lens wouldcorrespond, in this case, to varying the thickness of the imagingmedium.

The lenses illustrated in FIG. 4 are similar to those illustrated inFIG. 3 except that, in FIG. 4, each lens has one end face which isconvex about a radius of curvature lying on the lens axis, so that, inaddition to the variation in refractive index with radial distance fromthe lens axis, within the lens itself, there is a "conventional" lenseffect resulting from the refraction at the air/lens material interfaceat the convex end surface of the lens.

As disclosed in our co-pending Patent Application No. 8812644.6 an arrayof graded refractive index lenses may be formed in a light-transmittingsheet by using, as the sheet, a layer of a photopolymer material whichhas been selectively exposed optically to a pattern of dots. For afuller treatment of the process involved and of the theoretical andother considerations underlying graded refractive index GRIN lenses,reference should be had to that pending application.

It has now been found that there can be advantages in combining thegraded refractive index lens structure with features of conventionallens configuration to afford arrangements which act in a manneranalogous to conventional compound lens arrangements. Thus, referring toFIG. 4, a photopolymer layer 10 having formed therein an array of gradedrefractive index lenses with their axes perpendicular to the majordimensions of the sheet, as described generally in our co-pendingApplication No. 8812644.6, has additionally, on the surface of thephotopolymer layer remote from the photopolymer/substrate interface 12,a relief pattern comprising a plurality of convex domes 14, eachapproximating to a part-spherical surface having its centre of curvaturelying on the axis of a respective one of the graded refractive indexmicrolenses. Each such domed surface thus acts as a conventional convexlens surface, whereby the optical powers of the graded refractive indexand convex-surfaced components are added, affording microlenses ofenhanced power.

It is, of course, also possible, for example as a means of correctingaberration in the microlenses, in situations where such aberrations areof importance, to provide the graded refractive index layer 10 with asurface relief configuration affording a respective concave surfaceportion for each graded refractive index microlens, as illustrated inFIG. 5 so that the surface-refractive component of the lens has anegative power. Likewise, it is possible for each microlens to have agraded refractive index component affording a negative power, i.e.tending to act as a diverging lens, with the surface-refractivecomponent providing a counteracting positive power. In the last-notedtwo cases, the resulting combined effect may be either that of aconverging lens or that of a diverging lens. It is also possible, ofcourse, to have both the graded refractive index component and thesurface refractive component contribute respective negative opticalpower components to the combination, so that the latter operates as adiverging lens.

It has been found that a configuration such as described with referenceto FIG. 4 can be produced without the necessary for any distinctsurface-forming step, as a by-product of the selective polymerisationresulting from the selective exposure of the photopolymer (or rather,the photo-sensitive monomer). However, it will be appreciated that adesired surface-relief pattern may be imposed upon the photopolymerlayer, either before or after the selective polymerisation, by othermeans, for example by a mechanical moulding or pressing step, by anetching step or the like. Alternatively, the surface of the substrate towhich the polymer/monomer layer is applied, may be appropriatelyconfigured in relief, so that the desired surface-refractive effectsoccur at the polymer/substrate boundary, or, where the polymer issubsequently stripped off the substrate, at the surface of the polymerwhich was formerly in engagement with the substrate.

Whilst the preferred application of the invention is to the productionof an array of microlenses in a photopolymer sheet, it will beappreciated that individual lenses on a substantially larger scale andhaving the same sort of surface configuration may be produced.

It will likewise be understood that, if desired, both end surfaces ofeach graded refractive index lens may be convexly or concavely curved asdesired.

FIGS. 6 to 8 relate to embodiments of the invention according to claims5 and 6 herein.

Referring to FIG. 6, a light-diffusing material is manufactured bycoating a stable transparent base sheet, for example of polyester, witha layer of a photopolymerisable resin, such as a polyacrylamide resin ofa type discussed in more detail in GB 2206979 and subsequently exposingthe photopolymerisable layer to ultraviolet light to form an array ofgraded refractive index (GRIN) lenses therein. In the productillustrated in FIG. 6, the selective exposure is achieved, not byutilising an optical mask, as in GB2206979, but by providing atransparent base sheet 110 with a surface configuration such that thebase sheet 110 itself comprises an array of microlenses and using thelast-noted array of microlenses to vary selectively the intensity of theexposure light from point to point in the photopolymer. Thus, in thearrangement illustrated, one surface of the base sheet 110 comprises aclose-packed array of minute convexly domed regions whereby an array ofconverging microlenses is formed. In the example illustrated in FIG. 6,the layer 112 of photopolymerisable resin is applied to the oppositesurface of the base sheet 110 from the embossed surface. The product isexposed to light from the side afforded by the base sheet 110, suchlight being directed substantially at right angles to the general planeof the composite material and the exposure to light being substantiallyuniform over the material. However, due to the converging effect of theindividual convexly domed surface regions, the light striking each suchregion is converged, as indicated at (a) in FIG. 6, towards the opticalaxis of the microlens defined by each such region, whereby theunderlying photopolymer layer 12 is exposed to a higher intensity oflight, closer to such axis than further therefrom. As a result, thedegree of polymerisation varies with the distance from such axisproducing, beneath each domed surface region, a corresponding gradedrefractive index (GRIN) lens in the layer 112. Thus, as illustrated at(b) in FIG. 6, the converging effect of each domed region of the layer110 will be enhanced by the converging effect of the underlying (asviewed in FIG. 6) graded refractive index microlens.

Whilst, in the above description with reference to FIG. 6, it has beenassumed that the ultraviolet light used for the exposure strikes thesheet material substantially at right angles thereto, in somecircumstances it may be desired to illuminate the material from alocalised source placed above the material, so that the gradedrefractive index microlenses formed at locations progressively furtherfrom the point where the perpendicular to the sheet from the localisedlight source intersects the sheet will be progressively angled, in amanner similar to that described in relation to FIG. 12 in GB2206979.

It will be appreciated that, if desired, the base sheet 110 may beembossed or otherwise formed with an array of part-spherical concavehollows, resulting in an array of concave or diverging microlenses,whereby the exposure of the base sheet/polymerisable layer composite toultraviolet light through the base sheet 110 will produce acorresponding array of diverging graded refractive index microlenses.

It will also be appreciated that it is possible, as illustrated in FIG.7, to emboss or mould the photopolymerisable layer 112, rather than thebase sheet 110, with an array of domed regions and to expose thephotopolymerisable layer from the side of the light remote from the basesheet 110, the surface-refractive effect of the domed portions of theexposed surface of the layer 112 being relied upon to produce thedesired concentration of light towards the axis of each microlens sothat the consequent variation in graded refractive index enhances theeffect of refraction at the surface of the layer 112 for each of theresulting microlenses.

As shown in FIG. 8, it is also possible to configure the surface of thebase sheet 110 remote from the photopolymerisable layer 112, in a mannerknown per se, as a Fresnel lens or prism and to form the gradedrefractive index microlenses in the layer 112 by exposure, from the sideremote from the base sheet 110, through an appropriate mask, in the sameway as disclosed in GB 2206979, to form an array of graded refractiveindex microlenses in the layer 112, whereby the product has the effectof a combined converging (or diverging) lens or prism with a diffusingscreen, to afford characteristics particularly suited to a particularsituation of use, for example as a back projection screen.

FIG. 9 relates to an embodiment of the invention according to claim 7herein.

FIG. 9 illustrates schematically a scheme for carrying out the method,the photopolymer/monomer layer being indicated at 210, the optical maskat 216, the quarter wave plate at 212, and the source of laser light at214.

Alternatively, the photopolymer layer may be arranged to be exposedholographically, using a technique similar to that disclosed inpublished U.K. Patent Application GB2193344A, for exposing siliconslices, holographically to desired circuit patterns by reconstructing onsuch slices the image of the desired circuit patterns by illumination,by means of a reference laser beam, a hologram formed in a variablerefractive index layer applied to the hypotenuse face of a rightangle/45° prism. In that technique, the reference beam is directedthrough one of the two perpendicular faces of the prism, to beinternally reflected at the outer surface of the layer of variablerefractive index material and emerge from the other of the twoperpendicular faces, whilst light from the hologram passes through saidouter surface of the variable refractive index layer to form the desiredreconstructed image, on a silicon slice disposed parallel with saidhypotenuse face but spaced therefrom.

In the presently proposed variation of technique of GB 2193344A to formmicrolenses in a photopolymer layer, the photopolymer layer issubstituted for the silicon slice, and the hologram on the hypotenuseface of the prism is one formed by exposure of the variable refractiveindex material on the hypotenuse face under the conditions described inGB2193344A but replacing the mask in the form of an integrated circuitwith a mask in the form of a clear dot screen.

In practice, using the modified technique of GB 2193344A, only arelatively small area of the microlens screen can be exposed at a time,and so it is preferred to scan the photopolymer layer, exposingsuccessive small "blocks" of the surface of the layer at a time, thelaser source and prism, with the variable-refractive index hologrambeing constructed as a unit and said unit being "scanned" stepwise backand forth over the photopolymer layer with the latter being advanced bythe width of one "block" at the end of each scan, and exposure of asingle "block" being made at each step during each "scan".Alternatively, the laser, prism and hologram may be kept stationary andthe photopolymer layer "scanned" stepwise past the prism. In eithercase, the photopolymer layer is exposed on a block-by-block basis. Therotating quarter wave plate in each case is disposed to intercept thelaser light before it strikes the photopolymer. In principle, therotating quarter wave plate may be interposed at any location betweenthe laser and the photopolymer but is most conveniently positioneddirectly adjacent the exit window of the laser or directly in the pathof the laser beam immediately emerging from the laser.

The method according to the invention allows a substantial improvementto be obtained in the quality of microlens screens produced bylaser-exposure of photopolymeric material, the latter technique beingitself the preferred technique for forming such screens and the onlypractical technique to date for producing such microlens screens inwhich the optical axes of the individual microlenses are angled withrespect to the normal to the plane of the screen.

FIGS. 10 and 11 relate to an embodiment of the invention according toclaim 8 herein.

Referring to FIG. 10 of the accompanying drawings, the reference numeral310 indicates the right-angled prism and the reference 312 indicates thephotopolymer layer on the hypotenuse face of the prism 310, which layer312 bears the hologram derived from the mask referred to above (i.e. themask formed with said array of transparent "holes"). The prism 310, withlayer 312 adhered thereto, is mounted in an appropriate head, indicatedschematically at 314, so that the layer 312 forms the planar underfaceof the head 314 or part of a planar underface of the head 314. Thereference numeral 316 indicates schematically a source of laser lightarranged within the head 314 to direct laser light through theappropriate one of the two mutually perpendicular faces of the prism topass, internally of the prism, to the hypotenuse face, coated with layer312. The head 314 is mounted with its planar underface directly aboveand spaced only slightly from a layer 318 of a photopolymer, (or of themonomer from which the polymer will be derived by photo-polymerisation)and which layer 318 may be supported on and adhered to a transparentsubstrate 319, for example of a stable plastics material. The layer 318is parallel with the underface of the head 314 and is spaced therefromby a distance corresponding with the spacing of the original mask fromthe layer 312 during forming of the hologram. Accordingly, there isreproduced in the layer 318 an accurate image of the desired array ofilluminated spots, in laser light from the source 316, by virtue of thehologram incorporated in the layer 312, by virtue of the hologramincorporated in the layer 312, whereby a rectangular area or block ofthe photopolymer layer 318 directly below and corresponding in shape andsize to the hypotenuse face of the prism 310 is exposed to the desiredarray of light spots, whereby, because of the resulting selectivepolymerisation of the material of the layer 318, the desired array ofgraded refractive index microlenses is produced in the respective"block". The head 314 is mounted, by means indicated only schematicallyat 320, so that it can be traversed, in a horizontal plane, about thestrips 318, 319.

By way of illustration, the substrate 319 and photopolymer layer 318 maybe in the form of a continuous strip which is passed stepwise,longitudinally, in the direction of arrow A in FIG. 11 under the head314 whilst the head 314 may be so mounted as to allow it to be movedstepwise transversely of the strip, as indicated by arrow B in FIG. 11.According, for each longitudinal position of the strip 318, 319, thehead 14 may be traversed stepwise across the strip, with the underlying"block" of the photopolymer layer 318 being exposed at each step, sothat, as indicated in broken lines, a row of such "blocks" 324 will beexposed during one complete traverse of the head 314. The strip 318, 319may then be advanced by a distance corresponding to the longitudinalextent of each "block" 324 and the head 314 again traversed across thestrip to expose a further row of "blocks" and so on, whereby thephotopolymer layer 318 may be exposed block 324 by block 324, in ascanning mode. Thus, given a strip 318, 319, of the width envisaged forlarge-scale rear-projection television screens in a single exposure, thetechnique of the present invention allows a significant reduction in thenumber of discrete exposure steps required, as compared with aspot-by-spot (i.e. lens-by-lens) exposure system.

FIGS. 12 to 16 relate to an embodiment of the invention according toclaim 10 herein.

It is well known that the processing of silver halide in gelatin layerscan produce relief images by two main methods:

(a) the development of the layer followed by fixing--thus causing acollapse of the layer in the unexposed regions;

(b) the development of the layer followed by etch bleaching to dissolvethe developed silver and the gelatin in the regions of the exposedimage.

In preferred embodiments of the invention, method (b) is used to providea surface relief pattern which can be transferred to a transparentplastics sheet to afford a desired optical microstructure.

In the following, a procedure is outlined whereby an opticalmicrostructure, in the form of an array of spherical or quasi-sphericalmicrolenses in a transparent plastics sheet may be produced.

This procedure utilises a high resolution photographic material (e.g.Agfa-Gavaert 8ES6HD or Millimask material) comprising a (relativelythick about 6 μm) layer of silver halide/gelatin emulsion on asupporting substrate.

Referring to FIG. 12, in which the supporting substrate, for exampleplastics film or glass, is indicated at 510 and the emulsion layer at512, the material is first exposed to a light pattern, comprising anarray of tiny circular spots (e.g. of 18 m diameter) by contact printingwith a mask 514.

The contact lithographic method illustrated in FIG. 12 is best performedusing a chromium mask 514 created by a direct recording of the initialpattern of microscopic circular dots into a resist layer on a thin layerof metallic chromium on a transparent substrate followed by etching ofthe chromium to form microscopic holes or "windows". Such a chromiummask is durable and is cable of very high edge definition with respectto the microscopic holes or "windows".

The exposed photographic material is then developed in developer thataffords good depth development of the silver image. FIG. 13 illustratesthe material at the end of this stage, the reference 515 indicating aclump of silver grains representing the image of one of the apertures or"windows" in the mask 514.

The developed material is then etch bleached to produce etch pits 516 inthe gelatin layer, in the regions previously occupied by the spots ofsilver grains, as illustrated in FIG. 14. The etch bleaching may becarried out using a silver solvent solution such as acidified coppersulphate with the addition of potassium bromide and hydrogen peroxide.As a result of the use of the chromium mask with its array ofdefect-free holes with exceptional edge definition, the etched pits 516at the end of this etching stage are unusually clean and are devoid oftaper problems.

As illustrated in FIG. 15, a deformable plastics intermediate material518 is then applied to the relief surface using heat and or pressure.

The deformed plastics layer 518 is then stripped from the gelatin layer,the plastics layer thus affording an array of protuberancescorresponding to the array of etch pits in the gelatin layer. Theplastics layer may, at this stage, be briefly raised to its softeningtemperature so that the surfaces of the protuberances are "rounded off"by the inherent surface tension of the material whereby eachprotuberance adopts a part-spherical form.

After cooling the plastics material the deformed surface is used as a"master" for an electroforming or electrotyping step, illustrated inFIG. 16, in which a layer of nickel 520 is formed in intimate contactwith the surface of the plastics material 518 bearing said array ofprotuberances, the nickel layer 520 thus having a surface exactlycomplementary with that of the plastics layer and thus being formed withan array of concave pits. This surface of the nickel layer 520 is usedas the embossing surface of a durable embossing tool. By way of example,the nickel layer 520, once the plastics layer has been strippedtherefrom, may be bonded to the surface of a metal roller, to form anembossing roll, whereby transparent plastics sheet material may beembossed, to form an array of spherical microlenses thereon, by passingthe plastics sheet in continuous strip form, in a heated and softenedcondition, between such embossing roller and a counter-roller.

It will be appreciated that by utilising a mask 514 with the appropriate"image" for the contact printing phase, optical microstructures of otherforms besides arrays of spherical lenses may be formed. For example, aseries of parallel cylindrical lenses could be similarly formed, or,conceivably, structures such as Fresnel lenses or prisms. Similarly, byarranging for the final product to be reflective rather thantransparent, for example by metallisation of the product, correspondingreflective optical microstructures could be produced, such as arrays ofmicro-mirrors.

Furthermore such a method could be applied to the formation of pittedsurfaces of predetermined form for compact disc applications, forexample.

In a further variant, the plastics material to which the desired reliefpattern is imparted may be a photopolymerisable substance which issubsequently selectively exposed to polymerising radiation (e.g. toultra-violet light) in the region of each protuberance to bring aboutselective polymerisation with an associated refractive index variation,thereby adding a graded refractive index (GRIN) lens effect to eachmicrolens.

The method described above may utilise any of a number of commerciallyavailable high-resolution photolithographic emulsions, having emulsionthicknesses typically in the region 6 μm-17 μm. Generally it ispreferred to have microlenses with a diameter of around 3 times the lensthickness, possibly rather more, giving, for the above thickness range,a preferred microlens diameter range of 18 μm to 51 μm. The method may,of course, utilise emulsions of greater or lesser thickness withcorrespondingly greater or lesser preferred microlens diameter.

The developing and etching treatment of conventional photographicemulsions utilised in the above-described method in accordance with theinvention may also be used to achieve many effects, particularly on amicroscopic scale which have previously been associated with the use ofconventional photo-resists, with the advantage of much improved(photographic) speeds.

FIGS. 17 to 19 relate to an embodiment of the invention according toclaim 11 herein.

Referring to FIG. 17, a computer-controlled VDU has a cathode-ray tubeaffording a screen 610. The screen 610 is notionally divided into a gridof rows and columns of individual areas or "pixels", for example 200rows of 200 columns although, for the purposes of illustration, farfewer are shown in FIG. 17. Applied to the face of the screen 610 is asystem of light guides, indicated schematically at 612 in FIG. 17, (andillustrated in FIG. 17, for the purposes of illustration only, as beingsubstantially spaced from the screen 610). Each of the light guides 612is assigned to a particular one of the pixels on the screen 610 and hasan entrance end 612a which ideally extends over the whole of therespective pixel on the screen 610 and an exit end 612b which terminatesat the surface of a photosensitive sheet 614. The light guides 612 may,in principle, be individually formed, for example from bundles of glassfibres, flexible plastic elements or the like, but in the preferredembodiment, the light guides 612 are formed in a unitary sheet ofvariable refractive index polymeric material by appropriate systems ofrefractive index variations in the sheet.

The exit end 612b of each light guide 612 is of very smallcross-sectional area, for the size of such exit end determines theresolution which the printing system is capable of producing. Althoughin FIG. 17, for the purposes of illustration, the ends 612b areillustrated as being spaced apart transversely, the exit ends 612b areactually packed as close together as possible in a single row extendingwidth-wise across the photosensitive sheet 614.

The sheet 614 may be in the form of a continuous strip driven by adriving mechanism (not shown) and guided by appropriate rollers 616 forlongitudinal advance through the exposure zone in which is located therow of exit ends 612b of the light guides 612. The mechanism foradvancing the strip 614 is also controlled by the computer controllingthe illumination of the individual pixels of the screen 610 so that, forexample, for every field scan of the cathode-ray tube the strip 614 isadvanced by an amount corresponding to the dimension of each exit end612b. During each field scan, the computer determines, for each pixel,whether that pixel is to be illuminated or not and controls the"display" on the screen 610 accordingly. As a result, the surface of thestrip 614 is exposed or unexposed on a dot-by-dot basis in a grid ofrows and columns of such dots, to produce a desired image on the strip614. The apparatus may thus, for example, print lines of text, with eachcharacter being produced on a dot-matrix system, so that if, forexample, an 8×8 matrix is used, eight advancement steps of the strip 614will be required for each line of text and so on. It will be appreciatedthat the screen 610 is used purely as a mechanism for applyingindividual pulses of light to the respective light guides and that,accordingly, the image printed on the sheet 614 is not, in any normalsense, the same as the "image" appearing on the screen 610. Because thecathode ray tube follows a predefined scan and the light guides 612 arein a predetermined arrangement, it is possible to take advantage of thefact that the illumination to be applied in any one cycle to theentrance end of any light guide (assuming that that light guide is to beilluminated in the cycle) takes a very short time (corresponding to thetime taken by the cathode-ray tube to scan the lines upon which theentrance end of the respective light guide 612 is superimposed) bydisplacing the exit ends 612b of the respective guides relative to eachother, in the conveying direction of the strip 614, according to thesequence in which the light guides 612 are illuminated in the course ofa scan, to minimise or even eliminate the period between successiveadvance steps of the strip 614 for which the strip must remainstationary. It will be appreciated that, if the light guides 612 areappropriately arranged, such displacement of the outlet ends 612b in thedirection of conveyance of the strip 614 may be effected simply byinclining the row of exit ends 612b slightly relative to theperpendicular to the conveying direction.

The photosensitive strip 614 may comprise a conventional photographicmaterial which is appropriately processed after passing through theexposure zone or may comprise any of a number of suitable substitutesfor conventional photographic materials, and may be a monochrome orcolour printing material. It will be appreciated that instead of a stripof sheet material, a photosensitive drum of an electrostatic-typephotocopier may be arranged to rotate past the outlet ends of the lightguides 612 with a corresponding visible image being printed upon sheetsof paper in the manner known from such photocopiers. Various otheralternatives will be evident to those skilled in the art.

As indicated above, the light guides 612 are preferably formed in anintegral sheet of variable refractive index material, such as aphotopolymer, by appropriate localised variations in the refractiveindex in the sheet. It is a relatively straightforward matter toproduce, in such a sheet, a system of parallel light guides extending toone edge of the sheet, by exposure of such a sheet of photopolymer (orrather of the corresponding monomer) to a pattern of light comprising acorresponding system of bands of light with interposed bands ofdarkness. In such a system of light guides, light can be guided downeach of the relatively high refractive index strips by total internalreflection or analogous refraction at or in the boundary regions withthe relatively low refractive index zones on either side and iscorrespondingly internally reflected at the photopolymer/air surfaces ofthe sheet.

In FIGS. 18 and 19, the dotted regions represent the regions of lowerrefractive index in the photopolymeric sheet, and which act as theindividual light guides 612. FIG. 19 illustrates schematically howrefraction at the boundaries with the regions of lower refractive indexacts to contain light beams within the higher refractive index regions.The solid lines in FIG. 18 represent the upper and lower surfaces of thesheet where total internal reflection occurs to return to the guides 612light rays proceeding at an angle to such surfaces.

More complex techniques may be required to guide the light emerging fromthe individual pixels of the screen 610, principally at right angles tothe surface of the screen and thus the surface of the photopolymer sheetapplied thereto, into a direction or directions parallel with thesurface of the sheet and along the portions of the light guide runningparallel with such surfaces. Thus, for example, it may be necessary toresort to some form of "etching" or "diffusion" technique to produce avariation of refractive index with depth in the portion of thephotopolymer sheet applied to the screen 610, or to emboss or otherwiseform one or both surfaces of the sheet in the area applied to the screen610 to achieve the desired effect. It will be appreciated that theproblem of leading the remaining portions of the light guides 12 past orbetween the inlet ends applied to the individual pixels on the screen610 is similar to that arising in disposing the various electricalconductors around and connected with individual segments ofmulti-segment or multi-pixel LCD displays, for example, and is similarlysoluble. It will be appreciated that, if necessary, a separately formedoptical system may be provided, attached to the screen 610, forcollecting light from the individual pixels of the screen 610 anddelivering such light at respective outlets in a row of outlets to whichcan be applied one edge of a photopolymer sheet having light guidesextending longitudinally in the sheet, parallel with the surfacesthereof, from the last-mentioned edge to the edge which is positionedadjacent the photosensitive material 614, such light guides being formedby variations of the refractive index transversely of the sheet asdiscussed above, whereby the photopolymer sheet merely functions as theoptical equivalent of a flat-ribbon conductor in electronicsapplications.

It will be appreciated that since all of the pixels on the screen 610are effectively mapped into one row of pixels of the image formed on thephotosensitive material 614, a relatively coarse pixel resolution on thescreen 610 is adequate to provide a very fine resolution in the"printed" image on the medium 614 or in the visual image derivedtherefrom. Thus, for example, with an array of 200 rows by 200 columnsof pixels on the screen 610 (i.e. a total of 40,000 pixels) thecorresponding image on the strip 614 will comprise 40,000 dots from edgeto edge.

A microlens screen as described above, or as described in GB2206979A,may be used, in conjunction with an LCD pixelated screen used as abuilt-in monitor or "viewfinder" in a television camera or "camcorder"in place of the more conventional miniature crt. In such an application,the microlens screen will not normally serve as a projection screen toreceive an optically projected image of the picture on the LCD screenbut serves rather as a type of "Fourier filter" to depixelate the LCDscreen, (i.e. to render the boundaries of the pixels, or the transitionsfrom pixel to pixel, invisible to the eye). In an arrangement of thissort it is preferable to fill the space between the microlens screen andthe LCD screen by a colourless clear liquid of a refractive indexapproximating that of the microlens screen and the boundary plates ofthe LCD screen, for example a liquid such as glycerine. This expedienthas been found to eliminate certain unwanted interference effects suchas fringing or "Newtons" rings.

We claim:
 1. A sheet of transparent material comprising a layer of aphotopolymer which has been formed with an array of integral gradedrefractive index microlenses by selective exposure of aphotopolymerisable monomer to polymerising radiation wherein eachmicrolens terminates, on at least one of the surfaces of the sheet, in asurface relief formation which adds to the power of the respectivemicrolens.
 2. A method of making a light diffusing material comprisingproviding a sheet of light-transmitting base material, having a surfacethereof configured to form an array of microlenses, coating the basematerial with a layer of a medium, said medium being aphotopolymerisable monomer, of which the refractive index can be variedby exposure to light, then exposing the variable refractive index mediumto light through the said base material having said microlenses formedthereon, whereby the localised variation in intensity of said light,through the variable refractive index medium, produced by saidmicrolenses, causes corresponding variations in refractive index in saidmedium whereby there is produced, in said medium, an array of gradedrefractive index lenses each aligned with, and adding its optical effectto, a respective said microlens of the base material.
 3. A method ofmaking a light diffusing material comprising forming a sheet or layer ofa medium, said medium being a photopolymerisable monomer, of which therefractive index can be varied by exposure to light, one surface of saidsheet or layer being moulded or embossed to afford form an array ofmicrolenses, exposing the sheet or layer of said medium to light throughsaid moulded or embossed surface, whereby the localised variation insaid light, through the variable refractive index medium, produced byrefraction at said moulded or embossed surface, causes correspondingvariations in refractive index in said medium, whereby there isproduced, in said medium, an array of graded refractive index lenseseach aligned with, and adding its optical effect to, a respectivemicrolens surface defined by said moulding or embossing.
 4. A method ofmaking a light diffusing material comprising forming a sheet or layer ofa medium, said medium being a photopolymerisable monomer of which therefractive index can be varied by exposure to light, exposing the sheetor layer of said medium to light through an optical mask consisting ofan array of light transmitting areas in an opaque field whereby thelocalised variation in said light, through the variable refractive indexmedium, produced by said mask causes corresponding selectivepolymerisation of said medium in said light transmitting areas withcorresponding variations in refractive index in said medium, wherebythere is produced, in said medium, an array of graded refractive indexlenses, the method including leaving at least one surface of said sheetor layer free during exposure and photopolymerisation, whereby saidsheet or layer undergoes a surface modification associated with saidselective polymerisation to produce an array of microlenses defined bythe surface contour of said surface, each of said microlenses defined bysaid surface contour being aligned with, and adding its optical effectto, a respective said graded refractive index lens.